Demand for small and light-weight piezoelectric film resonators and filters configured with such devices is growing because of a rapid spread of wireless devices, a key example of which is a mobile phone. While dielectric filters and surface acoustic wave (SAW) filters have mainly been in use up to now, filters made up of a piezoelectric thin film resonator, which is a device offering good characteristics especially at high frequencies and enabling small form factors and monolithic integration, have recently been attracting interest.
FBARs (film bulk acoustic resonators) and SMRs (solidly mounted resonators) are included among such piezoelectric thin film resonators. An FBAR includes an upper electrode, piezoelectric film, and lower electrode on a substrate. A cavity is opened underneath the lower electrode at a portion where the upper electrode and the lower electrode face opposite each other. Here, the cavity is created by wet etching a sacrificial layer formed on the surface of the substrate, on which the lower electrode is placed, or by wet etching or dry etching, for example, the substrate from the back side. In an SMR, in lieu of the aforementioned cavity, an acoustic reflection film is created with films having high acoustic impedance and low acoustic impedance and laminated one after next with the film thicknesses of λ/4, where λ is an elastic wave wavelength.
When a high frequency voltage, which is an electrical signal, is applied between the upper electrode and the lower electrode of the piezoelectric thin film resonator, an elastic wave is generated as a result of a reverse piezoelectric effect in the piezoelectric film that is sandwiched between the upper electrode and the lower electrode. Furthermore, a strain created by the elastic waves is converted into an electrical signal by the piezoelectric effect. Because such elastic waves are totally reflected at the surfaces where the upper electrode film and the lower electrode film, respectively, are in contact with the air, they become vertically oscillating waves with the main direction of displacement along the thickness direction of the piezoelectric film. It is possible to obtain an resonator (or a filter formed with a plurality of resonators that are connected) having prescribed frequency characteristics, by taking advantage of such resonance phenomenon.
For example, an FBAR has a resonance at a frequency at which H, which is a total film thickness of the laminated structural portion made up mainly of the upper electrode film, piezoelectric film, and lower electrode film, formed over the cavity, equals an integral multiple (n times) of a half of the elastic wave wavelength λ (wavelength/2) (H=nλ/2). When the elastic wave propagation speed, which is determined by the piezoelectric film material, is V, the resonance frequency F is given by nV/(2H). Therefore, the resonance frequency F can be controlled by the total film thickness H of the laminated structure.
In general, such a piezoelectric thin film resonator and a device such as a filter formed with a plurality of resonators that are connected are manufactured as follows. Firstly, a large number of the aforementioned devices is formed with a single process on a wafer, and the wafer is ultimately diced, so that the individual prescribed chips, which include the aforementioned device, are obtained.
As described above, the resonance frequency (or the center frequency in the case of a filter) of a piezoelectric thin film resonator or a filter using the resonator is determined by the total film thickness of the laminated structure. For this reason, the resonance frequency (or in the case of the filter, the center frequency) shifts with the film thicknesses of the lower electrode film, piezoelectric film, and upper electrode film, which are the main films that make up the piezoelectric thin film resonator. For this reason, the resonance frequencies (or the center frequencies) of the piezoelectric thin film resonator formed in large numbers on a wafer and the filters formed by a plurality of connected such resonators vary in accordance with the distribution of the aforementioned film thicknesses across the wafer surface.
Because this variability in the resonance frequencies (or the center frequencies) leads to lower device yields, it is necessary to adjust the frequency variability across the wafer surface. Conventionally, adjustments have been made with a reduction in the film thicknesses of the lower electrode film, piezoelectric film, and the upper electrode film, which are the main component films, with etching (to shift the frequencies to higher frequencies), or with an increase in the film thicknesses with an addition to the upper electrode (to shift the frequencies to lower frequencies). Or a method is utilized in which a frequency adjusting film is newly formed in addition to the aforementioned main component films, and the adjustment is made with an increase or a decrease in the thickness of this frequency adjusting film (see, for example, Patent Documents 1 through 5 referenced below).    Patent Document 1: Japanese Patent Application Laid Open Publication No. 2002-299979    Patent Document 2: Japanese Patent Application Laid Open Publication No. 2002-299980    Patent Document 3: Japanese Patent Application Laid Open Publication No. 2002-335141    Patent Document 4: Japanese Patent Application Laid Open Publication No. 2002-344270    Patent Document 5: Japanese Patent Application Laid Open Publication No. 2005-286945